Department of Pharmacology and Beckman Institute, University of Illinois, Urbana-Champaign, Ill., USA.

3

Brain Research Institute, Niigata University, Niigata, Japan.

4

Department of Psychiatry, Johns Hopkins University School of Medicine, Baltimore, Md., USA.

Abstract

The corticostriatothalamic circuit regulates learning behaviors via dopamine neurotransmission. D2 long (D2L) receptors are an isoform of dopamine D2 receptors (D2Rs) and may act mainly at postsynaptic sites. It is well known that D2Rs influence high brain functions, but the roles of individual D2R isoforms are still unclear. To assess the influence of D2L receptors in visual discrimination learning, we performed visual discrimination and reversal tasks with D2L knockout mice using a touchscreen operant system. There were no significant differences in an operant conditioning task between genotypes. However, D2L knockout mice were impaired in both visual discrimination and reversal learning tasks. D2L knockout mice were also significantly slower than wild-type mice in collecting the reward in the visual discrimination task. These results indicate that D2L receptors play an important role in visual discrimination and reversal learning.

KEYWORDS:

a Touchscreen operant chamber. Condensed milk as a reward was dropped from the liquid dipper to the reward tray at the rear wall. b Schematic representation of visual stimuli and reward patterns during each stage of the experiment. Following habituation and pretrainings, the mice performed the operant conditioning, VD, and VDR tasks in sequence. The images below the stimuli indicate the outcome (reward vs. no reward) of touch responses to either CS. In the operant conditioning task, the CS was randomly presented on one side of the screen, and the other side of the screen was left blank. In the VD and VDR tasks, a pair of visual discriminative stimuli (CS1 and CS2) was presented on the screen. The left-right arrangement of the stimuli was determined pseudorandomly.